Method of electroplating plastic articles



METHOD OF ELECTROPLATING PLASTIC ARTICLES Abraham Piza Mendes, New York,N. Y.

No Drawing. Application April 26, 1956 Serial No. 580,964

6 Claims. (Cl. 204-20) My invention relates to the metal plating ofnon-metab lic substances and has particular relation to the plating ofplastic objects with gold or other precious metal utilizing avapor-vacuum plating procedure as an intermediate step.

The use of a vapor vacuum chamber for the coating of molded plasticarticles with such metals as aluminum or the like is well known andcommonly employed in the manufacture of inexpensive novelties, etc. Suchprocess deposits 21 very fine film of metal on the molded plastic,giving the coated article the appearance of being made of solid metal.With conventional methods of plastic molding it is possible toincorporate line and ornate detail of surface configuration in themolded plastic article, and the thin film of metal deposited by thevapor-vacuum method of plating, retains this fine detail.

It is also well-known to apply gold plating to metals by theconventional electrolytic or gold salts dip methods. Such plating iswidely used in the manufacture of costume jewelry, for example, to coata cast base of copper or bronze with gold. It has been found that fineand ornate surface details cannot be achieved in the case of the metalbase, and what detail there may be is appreciably lost when the goldplating is applied.

Because of the ability of plastics such as polystyrene to retain finesurface detail in molding, it would be extremely desirable to platemolded plastic articles with precious metals such as gold for themanufacture of costume jewelry or the like. Up to the present time,there has been no method found, to my knowledge, of plating moldedplastic articles directly with gold, because gold will not adhere toplastics nor form thereon by conventional gold-plating methods. Thedeposition of gold or other precious metals by the vapor-vacuum orsputtering methods is of course impractical commercially because of thetremendous Waste of precious materials by this method.

I have found that gold can be deposited on an article of plastic orother non-conductor of electricity, by first coating the article, as bythe vapor-vacuum process, with a metal close to gold in theelectromotive series. Gold is placed below hydrogen in the electromotiveseries and has a positive potential in normal metal ion solution; themetal to be applied by the vapor vacuum process must, have a positivepotential and it must also be capable of adhering to the surface of theplastic during the deposition of gold.

It is well known that in gold plating, copper alfords an excellentplating surface because of its ability to release metallic gold from theplating solution and to provide a surface to which the gold will adhere.Copper is therefore widely used both in electroplating and in goldsolution dipping without electricity. It was found, however, that when amolded, plastic article was .coated with a thin film of copper by thevapor-vacuum method, and then dipped in the gold solution, the copperpeeled otf without being replaced by gold. This was the result,

atent ice whether or not current was applied. When current was applied,wide variations in voltage made little difference. The tendency ofmetallic copper to go into solution, replacing gold ions in thatsolution, is greater than the adhesion of the metallic copper to thesurface with which it is in contact, even when the tendency to go intosolution is opposed by an electric current of such direction that itmoves metallic (positive) ions to that surface. Furthermore, it wasapparent that the copper film was so thin that, having partiallyreplaced the gold ions, there was not sutficient metallic copperremaining to enable the gold ions to lose their charges and become amolecular plating. The gold ions will not lose their charges in contactwith the plastic.

Antimony and bismuth which have positive electric potentials on theelectromotive table, were tried as base coats upon which to plate goldand have proved satisfactory. These metals were applied to the surfaceof a polystyrene article by the vapor-vacuum method, and a coating ofgold was then applied thereupon by electroplating. The antimony andbismuth fihns adhered to the surface of the plastic article throughoutthe electroplat ing process and also adhered well to the plated goldcoating. Gold can be deposited on these metals, however, only byelectroplating. Since arsenic is grouped with antimony and bismuth inthe electromotive series and has similar physical properties, it alsocould presumably be used, if adequate precautions be taken to preventthe formation of compounds deleterious to health.

I have found that the deposition of gold is more rapid, and a coating ofbetter appearance is produced if by the same vapor-vacuum procedure, acoating of copper is applied over the above-stated materials.Conventional electrolytic methods may then be used for the deposition ofgold thereon, but the voltage used is preferably lower than thatconventionally employed, since high voltages tend to tear or otherwisedamage the thin metal films deposited by the vapor-vacuum process.

A film of copper deposited by vapor vacuum over a thin film of metalclose to gold in the electromotive series may be used to obtain acoating of gold thereon by any of the conventional methods such as dip,salt bath, tumbling," etc. In these methods, the gold ions in solutionpartially replace the copper film, since no electric current is applied.

By this procedure, the base coating of metal acts as a bond between thecoating of copper and the surface of the plastic article, preventing thecopper film from peeling off in the gold solution. At the same time, thebase coating acts as a metal surface to enable the gold ions to losetheir charges. The gold ions will not lose their charges in contact withthe plastic surface.

Other metals were used experimentally for the base coating, but provedunsatisfactory. Lead was used, for example, producing good films on themolded plastic articles. Copper was then deposited on the lead basecoating by the same vacuum procedure, forming a fine film coating whichretained the surface configuration of the molded plastic. When thecoated plastic articles were then placed in a gold plating bath,however, the results were unsatisfactory. Adhesion of the lead to theplastic was insufficient, and copper was removed into the gold solutionwithout being replaced by gold. In the conventional electroplatingmethod, the copper film began to show plated gold at 1% volts, butloosened and began to peel from the plastic within ten to fifteenseconds.

Similar unsatisfactory results were achieved using tin and aluminum inplace of lead.

Thus the only metals which appear to serve adequately Patented May 13,1958 series, namely antimony, bismuth, and arsenic. These metals, incommon with copper and the noble metals (gold, silver and platinum); allhave positive potentials in contact with solutions in which the metalion .is normal. The metals, antimony, bismuth and. arsenic are directlyabove copper in the electromotive series, and are relaely'cl'ose togold. These metals, therefore, will not react with the. copper filmwhendipped' in the gold.

solution, so that adhesion to both plastic and copper is maintained, andthe coatings do not peel or loosen. In addition, arsenic,.antimony andbismuth have low melting points, so that they may be readily vaporizedin the vacuum chamber, and in addition are commercially available andinexpensive.

When arsenic, antimony, or bismuth is used to provide the base coatingon the plastic by the vapor vacuum method, it forms a fine adherentfilm. on the surface of the plastic. Copper is then plated over the basecoat by the vapor-vacuum method, forming a film which bonds strongly tothe base coat and completely covers the base coating. Since copper isused as the intermediate coat, a final gold plating may then be appliedeither by-the conventional dip. method or by conventional electroplatingmethods. In the, dip method, the gold ions replace the outer surface ofthe copper film and then plate the remainder of the copper film beforethe latter has gone completely into solution. The metal of the basecoatadheres to the remainder of the copper film and prevents it fromloosening or peeling.

In Principles of Electroplating and Electroforming by Blum and Hogaboom,published in 1924, the electromoh've series is set forth at page 344. Inthis series the approximate single potentials of metals toward solutionswith normal metal ion concentration, and with a standard based on thenormal hydrogen electrode as zero, are listed. The potential of antimonyis given as 0.19 volt, arsenic as 0.32 volt, bismuth as 0.33 volt, andcopper as 0.34 volt. The metals suitable for use as a base coat on theplastic article thus have positive potentials, but less than 034 volt,the potential of copper. From the potentials given, it would be apparentthat bismuth would yield better plating results than antimony, and testshave shown this to be true, although satisfactory results were achievedwith both bismuth and antimony.

The following is an example of a specific plating procedure in whichoptimum results were, achieved:

The plastic form was made of polystyrene and was first cleaned using acommercial: plastic cleaner; The surface of the plastic was then coveredwith a lacquer compounded to be compatible therewith. Such a lacquer ispresently commercially available for vacuum coating withv aluminum, and.provides better adhesion to the plastic as well as scaling in vaporproducing agents such as residual plasticizcr.

The metal -for producing the base coat was then applied to the plastic.For this purpose, the plastic article was placed in a vacuum chamber,and pure bismuth contained in a molybdenum crucible was also placed inthe chamber. The crucible was heated to a dull red. by the passage of anelectric current therethrough. As the bismuth vaporized, a good film wasformed on the plastic article, under a vacuum of approximately one-halfmicron.

A coating of copper was then formed over the bismuth film by the samevacuum chamber procedure just described, except that the vacuum employedto volatilize the copper was approximately micron, and the molybdenumcrucible was heated to white heat.

The coated article was then placed in a commercial electrolyte solutionof gold salts, and a voltage of 1.5 to 2.0 volts applied to theelectrodes. A plating of gold adhered to the copper film with thebismuth film asa base. The gold plate was suflicientl'y adherent towithstand polishing with silver polish.

A plastic article with a base coat of bismuth and a second coat ofcopper (applied by the vapor vacuum method) was also plated with gold bythe conventional salt water process. The article was placed in a porouscup containing a solution of gold salts. The porous cup was placed in alarger vessel containing a solution of sodium chloride and a strip ofzinc was suspended in the sodium chloride solution. Gold is thendeposited on the article which becomes. the cathode of a cell of whichthe anode was the zinc strip.

While the invention has been: described heretofore in termsof achievinga gold plating, it applies equally to a plating of any noble metal,namely silver, platinum or gold. In addition to polystyrene, articlesmade of other rigid plastics, such as acrylic resins, may be plated bythe methods described herein.

What is claimed is:

1. A method of coating a plastic article with. a noble metal, whichcomprises the steps of forming, onsaid plastic article in a vapor vacuumchamber a thin film base oil a metal selected from the group consisting.of antimony, bismuth and arsenic, and then coating said article withthev noble metal, by electrodeposition.

2. A method according to claim 1 in which said noble metal is gold.

3. A method of coating a non-porous plastic article with an outerplating of a noble metal, which comprises the steps of vapor depositingon said plastic article a thin film base coating of metal selected froma group consisting of arsenic, antimony and bismuth, then vapordepositiing over said base coating an adhering thin film coating ofcopper, and finally plating the copper coating with a coating of noblemetal by electrodeposition.

4. A method according to claim 3 in which the noble metal is gold.

5.. A method of coating a molded non-porous plastic article with anouter ornamental plating of a noble metal, which comprises the steps offorming on said plastic article in a vapor-vacuum chamber a thin filmbase coating of bismuth then forming over said base coating in avapor-vacuum chamber an adhering thin film coating of copper, andfinally plating-the copper coating with a coatingot, gold byelectrodeposition, the gold plating adhering to the copper coating.

6. A method of plating a molded plastic article having ornate surfaceconfigurations with an adhering outer ornamental coating of a noblemetal sufiiciently fine to retain the details of said surfaceconfiguration, said method comprising the steps of coating said plasticarticle by evaporation. with, a fine base coating, of a metal selectedfrom the group consisting of arsenic, antimony, and bismuth, thenapplying by evaporation an adhering intermediate conductive coating overthe base coating. the intermediate coating comprising a thin film ofcopper, and finally providing the plastic article with a thin outeradhering. coating of the noble metal by elcctrodep-- osition of thenoble metal on the intermediate coating.

References Cited in the file of this patent UNITED STATES PATENTS887,482 Lammers et al May 12, 1908 1,548,432 Belous Aug. 4, 19252,133,995 Lukens Oct. 25, 1938 2,333,534 Lang Nov. 2, 1943 2,383,311Hein Aug. 21, 1945 2,423,476. Billings et a1. July 8,, 1947 2,439,983-Morgan et al. Apr. 20, 1948 2,482,054 Colbert et al. Sept. 13, 19492,501,737 Porter Mar. 28, 1950 2,680,699 Rubin June 8, 1954 2,702,259Sommer Feb. 15, 1955

1. A METHOD OF COATING A PLASTIC ARTICLE WITH A NOBLE METAL, WHICHCOMPRISES THE STPS OF FORMING ON SAID PLASTIC ARTICLE IN A VAPOR VACUUMCHAMBER A THIN FILM BASE OF A METAL SELECTED FROM THE GROUP CONSISTINGOF ANTIMONY, BISMUTH AND ARSENIC, AND THEN COATING SAID ARTICLE WITH THENOBLE METAL BY ELECTRODEPOSITION.